Simplified adenine base editors improve adenine base editing efficiency in rice

Targeted genome-modification tools and their advanced applications in crop breeding

Crop improvement by genome editing involves the targeted alteration of genes to improve plant traits, such as stress tolerance, disease resistance or nutritional content. Techniques for the targeted modification of genomes have evolved from generating random mutations to precise base substitutions, followed by insertions, substitutions and deletions of small DNA fragments, and are finally starting to achieve precision manipulation of large DNA segments. Recent developments in base editing, prime editing and other CRISPR-associated systems have laid a solid technological foundation to enable plant basic research and precise molecular breeding. In this Review, we systematically outline the technological principles underlying precise and targeted genome-modification methods. We also review methods for the delivery of genome-editing reagents in plants and outline emerging crop-breeding strategies based on targeted genome modification. Finally, we consider potential future developments in precise genome-editing technologies, delivery methods and crop-breeding approaches, as well as regulatory policies for genome-editing products.

Precise fine-turning of GhTFL1 by base editing tools defines ideal cotton plant architecture

BACKGROUND

CRISPR/Cas-derived base editor enables precise editing of target sites and has been widely used for basic research and crop genetic improvement. However, the editing efficiency of base editors at different targets varies greatly.

RESULTS

Here, we develop a set of highly efficient base editors in cotton plants. GhABE8e, which is fused to conventional nCas9, exhibits 99.9% editing efficiency, compared to GhABE7.10 with 64.9%, and no off-target editing is detected. We further replace nCas9 with dCpf1, which recognizes TTTV PAM sequences, to broaden the range of the target site. To explore the functional divergence of TERMINAL FLOWER 1 (TFL1), we edit the non-coding and coding regions of GhTFL1 with 26 targets to generate a comprehensive allelic population including 300 independent lines in cotton. This allows hidden pleiotropic roles for GhTFL1 to be revealed and allows us to rapidly achieve directed domestication of cotton and create ideotype germplasm with moderate height, shortened fruiting branches, compact plant, and early-flowering. Further, by exploring the molecular mechanism of the GhTFL1L86P and GhTFL1K53G+S78G mutations, we find that the GhTFL1L86P mutation weakens the binding strength of the GhTFL1 to other proteins but does not lead to a complete loss of GhTFL1 function.

CONCLUSIONS

This strategy provides an important technical platform and genetic information for the study and creation of ideal plant architecture.

Progress in gene editing tools, implications and success in plants: a review

Genetic modifications are made through diverse mutagenesis techniques for crop improvement programs. Among these mutagenesis tools, the traditional methods involve chemical and radiation-induced mutagenesis, resulting in off-target and unintended mutations in the genome. However, recent advances have introduced site-directed nucleases (SDNs) for gene editing, significantly reducing off-target changes in the genome compared to induced mutagenesis and naturally occurring mutations in breeding populations. SDNs have revolutionized genetic engineering, enabling precise gene editing in recent decades. One widely used method, homology-directed repair (HDR), has been effective for accurate base substitution and gene alterations in some plant species. However, its application has been limited due to the inefficiency of HDR in plant cells and the prevalence of the error-prone repair pathway known as non-homologous end joining (NHEJ). The discovery of CRISPR-Cas has been a game-changer in this field. This system induces mutations by creating double-strand breaks (DSBs) in the genome and repairing them through associated repair pathways like NHEJ. As a result, the CRISPR-Cas system has been extensively used to transform plants for gene function analysis and to enhance desirable traits. Researchers have made significant progress in genetic engineering in recent years, particularly in understanding the CRISPR-Cas mechanism. This has led to various CRISPR-Cas variants, including CRISPR-Cas13, CRISPR interference, CRISPR activation, base editors, primes editors, and CRASPASE, a new CRISPR-Cas system for genetic engineering that cleaves proteins. Moreover, gene editing technologies like the prime editor and base editor approaches offer excellent opportunities for plant genome engineering. These cutting-edge tools have opened up new avenues for rapidly manipulating plant genomes. This review article provides a comprehensive overview of the current state of plant genetic engineering, focusing on recently developed tools for gene alteration and their potential applications in plant research.

CRISPR/Cas9-mediated genome editing techniques and new breeding strategies in cereals - current status, improvements, and perspectives

Cereal crops, including triticeae species (barley, wheat, rye), as well as edible cereals (wheat, corn, rice, oat, rye, sorghum), are significant suppliers for human consumption, livestock feed, and breweries. Over the past half-century, modern varieties of cereal crops with increased yields have contributed to global food security. However, presently cultivated elite crop varieties were developed mainly for optimal environmental conditions. Thus, it has become evident that taking into account the ongoing climate changes, currently a priority should be given to developing new stress-tolerant cereal cultivars. It is necessary to enhance the accuracy of methods and time required to generate new cereal cultivars with the desired features to adapt to climate change and keep up with the world population expansion. The CRISPR/Cas9 system has been developed as a powerful and versatile genome editing tool to achieve desirable traits, such as developing high-yielding, stress-tolerant, and disease-resistant transgene-free lines in major cereals. Despite recent advances, the CRISPR/Cas9 application in cereals faces several challenges, including a significant amount of time required to develop transgene-free lines, laboriousness, and a limited number of genotypes that may be used for the transformation and in vitro regeneration. Additionally, developing elite lines through genome editing has been restricted in many countries, especially Europe and New Zealand, due to a lack of flexibility in GMO regulations. This review provides a comprehensive update to researchers interested in improving cereals using gene-editing technologies, such as CRISPR/Cas9. We will review some critical and recent studies on crop improvements and their contributing factors to superior cereals through gene-editing technologies.

Advances in cis-element- and natural variation-mediated transcriptional regulation and applications in gene editing of major crops

Transcriptional regulation is crucial to control of gene expression. Both spatio-temporal expression patterns and expression levels of genes are determined by the interaction between cis-acting elements and trans-acting factors. Numerous studies have focused on the trans-acting factors that mediate transcriptional regulatory networks. However, cis-acting elements, such as enhancers, silencers, transposons, and natural variations in the genome, are also vital for gene expression regulation and could be utilized by clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9)-mediated gene editing to improve crop quality and yield. In this review, we discuss current understanding of cis-element-mediated transcriptional regulation in major crops, including rice (Oryza sativa), wheat (Triticum aestivum), and maize (Zea mays), as well as the latest advancements in gene editing techniques and their applications in crops to highlight prospective strategies for crop breeding.

Green revolution to genome revolution: driving better resilient crops against environmental instability

Crop improvement programmes began with traditional breeding practices since the inception of agriculture. Farmers and plant breeders continue to use these strategies for crop improvement due to their broad application in modifying crop genetic compositions. Nonetheless, conventional breeding has significant downsides in regard to effort and time. Crop productivity seems to be hitting a plateau as a consequence of environmental issues and the scarcity of agricultural land. Therefore, continuous pursuit of advancement in crop improvement is essential. Recent technical innovations have resulted in a revolutionary shift in the pattern of breeding methods, leaning further towards molecular approaches. Among the promising approaches, marker-assisted selection, QTL mapping, omics-assisted breeding, genome-wide association studies and genome editing have lately gained prominence. Several governments have progressively relaxed their restrictions relating to genome editing. The present review highlights the evolutionary and revolutionary approaches that have been utilized for crop improvement in a bid to produce climate-resilient crops observing the consequence of climate change. Additionally, it will contribute to the comprehension of plant breeding succession so far. Investing in advanced sequencing technologies and bioinformatics will deepen our understanding of genetic variations and their functional implications, contributing to breakthroughs in crop improvement and biodiversity conservation.

Cytosine base editors optimized for genome editing in potato protoplasts

In this study, we generated and compared three cytidine base editors (CBEs) tailor-made for potato (Solanum tuberosum), which conferred up to 43% C-to-T conversion of all alleles in the protoplast pool. Earlier, gene-edited potato plants were successfully generated by polyethylene glycol-mediated CRISPR/Cas9 transformation of protoplasts followed by explant regeneration. In one study, a 3-4-fold increase in editing efficiency was obtained by replacing the standard Arabidopsis thaliana AtU6-1 promotor with endogenous potato StU6 promotors driving the expression of the gRNA. Here, we used this optimized construct (SpCas9/StU6-1::gRNA1, target gRNA sequence GGTC4C5TTGGAGC12AAAAC17TGG) for the generation of CBEs tailor-made for potato and tested for C-to-T base editing in the granule-bound starch synthase 1 gene in the cultivar Desiree. First, the Streptococcus pyogenes Cas9 was converted into a (D10A) nickase (nCas9). Next, one of three cytosine deaminases from human hAPOBEC3A (A3A), rat (evo_rAPOBEC1) (rA1), or sea lamprey (evo_PmCDA1) (CDA1) was C-terminally fused to nCas9 and a uracil-DNA glycosylase inhibitor, with each module interspaced with flexible linkers. The CBEs were overall highly efficient, with A3A having the best overall base editing activity, with an average 34.5%, 34.5%, and 27% C-to-T conversion at C4, C5, and C12, respectively, whereas CDA1 showed an average base editing activity of 34.5%, 34%, and 14.25% C-to-T conversion at C4, C5, and C12, respectively. rA1 exhibited an average base editing activity of 18.75% and 19% at C4 and C5 and was the only base editor to show no C-to-T conversion at C12.

Challenges and potentials of new breeding techniques in Cannabis sativa

Cannabis sativa L. is an ancient crop used for fiber and seed production and not least for its content of cannabinoids used for medicine and as an intoxicant drug. Due to the psychedelic effect of one of the compounds, tetrahydrocannabinol (THC), many countries had regulations or bands on Cannabis growing, also as fiber or seed crop. Recently, as many of these regulations are getting less tight, the interest for the many uses of this crop is increasing. Cannabis is dioecious and highly heterogenic, making traditional breeding costly and time consuming. Further, it might be difficult to introduce new traits without changing the cannabinoid profile. Genome editing using new breeding techniques might solve these problems. The successful use of genome editing requires sequence information on suitable target genes, a genome editing tool to be introduced into plant tissue and the ability to regenerate plants from transformed cells. This review summarizes the current status of Cannabis breeding, uncovers potentials and challenges of Cannabis in an era of new breeding techniques and finally suggests future focus areas that may help to improve our overall understanding of Cannabis and realize the potentials of the plant.

Multi-faceted CRISPR-Cas9 strategy to reduce plant based food loss and waste for sustainable bio-economy - A review

Currently, international development requires innovative solutions to address imminent challenges like climate change, unsustainable food system, food waste, energy crisis, and environmental degradation. All the same, addressing these concerns with conventional technologies is time-consuming, causes harmful environmental impacts, and is not cost-effective. Thus, biotechnological tools become imperative for enhancing food and energy resilience through eco-friendly bio-based products by valorisation of plant and food waste to meet the goals of circular bioeconomy in conjunction with Sustainable Developmental Goals (SDGs). Genome editing can be accomplished using a revolutionary DNA modification tool, CRISPR-Cas9, through its uncomplicated guided mechanism, with great efficiency in various organisms targeting different traits. This review's main objective is to examine how the CRISPR-Cas system, which has positive features, could improve the bioeconomy by reducing food loss and waste with all-inclusive food supply chain both at on-farm and off-farm level; utilising food loss and waste by genome edited microorganisms through food valorisation; efficient microbial conversion of low-cost substrates as biofuel; valorisation of agro-industrial wastes; mitigating greenhouse gas emissions through forestry plantation crops; and protecting the ecosystem and environment. Finally, the ethical implications and regulatory issues that are related to CRISPR-Cas edited products in the international markets have also been taken into consideration.

Enhancing the quality of staple food crops through CRISPR/Cas-mediated site-directed mutagenesis

The enhancement of CRISPR-Cas gene editing with robust nuclease activity promotes genetic modification of desirable agronomic traits, such as resistance to pathogens, drought tolerance, nutritional value, and yield-related traits in crops. The genetic diversity of food crops has reduced tremendously over the past twelve millennia due to plant domestication. This reduction presents significant challenges for the future especially considering the risks posed by global climate change to food production. While crops with improved phenotypes have been generated through crossbreeding, mutation breeding, and transgenic breeding over the years, improving phenotypic traits through precise genetic diversification has been challenging. The challenges are broadly associated with the randomness of genetic recombination and conventional mutagenesis. This review highlights how emerging gene-editing technologies reduce the burden and time necessary for developing desired traits in plants. Our focus is to provide readers with an overview of the advances in CRISPR-Cas-based genome editing for crop improvement. The use of CRISPR-Cas systems in generating genetic diversity to enhance the quality and nutritional value of staple food crops is discussed. We also outlined recent applications of CRISPR-Cas in developing pest-resistant crops and removing unwanted traits, such as allergenicity from crops. Genome editing tools continue to evolve and present unprecedented opportunities to enhance crop germplasm via precise mutations at the desired loci of the plant genome.

Getting better all the time - recent progress in the development of CRISPR/Cas-based tools for plant genome engineering

Since their first adaptation for plant genome editing, clustered regularly interspaced short palindromic repeats/CRISPR-associated system nucleases and tools have revolutionized the field. While early approaches focused on targeted mutagenesis that relies on mutagenic repair of induced double-strand breaks, newly developed tools now enable the precise induction of predefined modifications. Constant efforts to optimize these tools have led to the generation of more efficient base editors with enlarged editing windows and have enabled previously unachievable C-G transversions. Prime editors were also optimized for the application in plants and now allow to accurately induce substitutions, insertions, and deletions. Recently, great progress was made through precise restructuring of chromosomes, which enables not only the breakage or formation of genetic linkages but also the swapping of promoters.

Plant base editing and prime editing: The current status and future perspectives

Precise replacement of an allele with an elite allele controlling an important agronomic trait in a predefined manner by gene editing technologies is highly desirable in crop improvement. Base editing and prime editing are two newly developed precision gene editing systems which can introduce the substitution of a single base and install the desired short indels to the target loci in the absence of double-strand breaks and donor repair templates, respectively. Since their discoveries, various strategies have been attempted to optimize both base editor (BE) and prime editor (PE) in order to improve the precise editing efficacy, specificity, and expand the targeting scopes. Here, we summarize the latest development of various BEs and PEs, as well as their applications in plants. Based on these progresses, we recommend the appropriate BEs and PEs for both basic plant research and crop improvement. Moreover, we propose the perspectives for further optimization of these two editors. We envision that both BEs and PEs will become the routine and customized precise gene editing tools for both plant biological research and crop improvement in the near future.

Precise genome editing with base editors

Single-nucleotide variants account for about half of known pathogenic genetic variants in human. Genome editing strategies by reversing pathogenic point mutations with minimum side effects have great therapeutic potential and are now being actively pursued. The emerge of precise and efficient genome editing strategies such as base editing and prime editing provide powerful tools for nucleotide conversion without inducing double-stranded DNA breaks (DSBs), which have shown great potential for curing genetic disorders. A diverse toolkit of base editors has been developed to improve the editing efficiency and accuracy in different context of application. Here, we summarized the evolving of base editors (BEs), their limitations and future perspective of base editing-based therapeutic strategies.

A CRISPR way for accelerating cereal crop improvement: Progress and challenges

Humans rely heavily on cereal grains as a key source of nutrients, hence regular improvement of cereal crops is essential for ensuring food security. The current food crisis at the global level is due to the rising population and harsh climatic conditions which prompts scientists to develop smart resilient cereal crops to attain food security. Cereal crop improvement in the past generally depended on imprecise methods like random mutagenesis and conventional genetic recombination which results in high off targeting risks. In this context, we have witnessed the application of targeted mutagenesis using versatile CRISPR-Cas systems for cereal crop improvement in sustainable agriculture. Accelerated crop improvement using molecular breeding methods based on CRISPR-Cas genome editing (GE) is an unprecedented tool for plant biotechnology and agriculture. The last decade has shown the fidelity, accuracy, low levels of off-target effects, and the high efficacy of CRISPR technology to induce targeted mutagenesis for the improvement of cereal crops such as wheat, rice, maize, barley, and millets. Since the genomic databases of these cereal crops are available, several modifications using GE technologies have been performed to attain desirable results. This review provides a brief overview of GE technologies and includes an elaborate account of the mechanisms and applications of CRISPR-Cas editing systems to induce targeted mutagenesis in cereal crops for improving the desired traits. Further, we describe recent developments in CRISPR-Cas-based targeted mutagenesis through base editing and prime editing to develop resilient cereal crop plants, possibly providing new dimensions in the field of cereal crop genome editing.

Rapid creation of CENH3-mediated haploid induction lines using a cytosine base editor (CBE)

Haploid induction (HI) can create true-breeding lines in a single generation, which can significantly accelerates the breeding process. In recent years, scientists have developed a variety of new techniques to induce haploids through manipulation of CENH3, a variant of the centromere-specific histone H3. One alternative approach is based on CENH3 point mutations derived from EMS/TILLING, which is not lethal and yet is responsible for inducing haploids. However, most residues have been obtained by EMS mutagenesis over a long period of time. Recently, a new approach called 'base editing' was developed for plants. Here, we report a new method that uses a cytosine base editor (CBE) to create a point mutation of CENH3 as a haploid induction line, which substitutes adenine (A) for guanine (G). As proof of the extreme simplicity of this approach to create haploid-induced lines, we identified an L130F substitution within the histone fold domain in Arabidopsis thaliana. Subsequently, we tested the haploid-inducing potential of homozygous L130F plants by pollinating them with Col-0, and obtained 2.9% paternal haploid plants. In brief, our innovative technology provides a new perspective for the promotion of CENH3-mediated haploid induction in crops, and also provides a variety of options for breeders. Such conserved point mutations as L130F could be developed into a general instrument for haploid induction in a wide range of plant species. Extending these systems would represent a major advance over haploid production.

Application of CRISPR/Cas system in cereal improvement for biotic and abiotic stress tolerance

Application of the recently developed CRISPR/Cas tools might help enhance cereals' growth and yield under biotic and abiotic stresses. Cereals are the most important food crops for human life and an essential source of nutrients for people in developed and developing countries. The growth and yield of all major cereals are affected by both biotic and abiotic stresses. To date, molecular breeding and functional genomic studies have contributed to the understanding and improving cereals' growth and yield under biotic and abiotic stresses. Clustered, regularly inter-spaced, short palindromic repeats (CRISPR)/CRISPR-associated protein (Cas) system has been predicted to play a major role in precision plant breeding and developing non-transgenic cereals that can tolerate adverse effects of climate change. Variants of next-generation CRISPR/Cas tools, such as prime editor, base editor, CRISPR activator and repressor, chromatin imager, Cas12a, and Cas12b, are currently used in various fields, including plant science. However, few studies have been reported on applying the CRISPR/Cas system to understand the mechanism of biotic and abiotic stress tolerance in cereals. Rice is the only plant used frequently for such studies. Genes responsible for biotic and abiotic stress tolerance have not yet been studied by CRISPR/Cas system in other major cereals (sorghum, barley, maize and small millets). Examining the role of genes that respond to biotic and abiotic stresses using the CRISPR/Cas system may help enhance cereals' growth and yield under biotic and abiotic stresses. It will help to develop new and improved cultivars with biotic- and abiotic-tolerant traits for better yields to strengthen food security. This review provides information for cereal researchers on the current status of the CRISPR/Cas system for improving biotic and abiotic stress tolerance in cereals.

Comprehending the evolution of gene editing platforms for crop trait improvement

CRISPR (Clustered Regularly Interspaced Short Palindromic Repeats)/Cas (CRISPR-associated) system was initially discovered as an underlying mechanism for conferring adaptive immunity to bacteria and archaea against viruses. Over the past decade, this has been repurposed as a genome-editing tool. Numerous gene editing-based crop improvement technologies involving CRISPR/Cas platforms individually or in combination with next-generation sequencing methods have been developed that have revolutionized plant genome-editing methodologies. Initially, CRISPR/Cas nucleases replaced the earlier used sequence-specific nucleases (SSNs), such as zinc-finger nucleases (ZFNs) and transcription activator-like effector nucleases (TALENs), to address the problem of associated off-targets. The adaptation of this platform led to the development of concepts such as epigenome editing, base editing, and prime editing. Epigenome editing employed epi-effectors to manipulate chromatin structure, while base editing uses base editors to engineer precise changes for trait improvement. Newer technologies such as prime editing have now been developed as a "search-and-replace" tool to engineer all possible single-base changes. Owing to the availability of these, the field of genome editing has evolved rapidly to develop crop plants with improved traits. In this review, we present the evolution of the CRISPR/Cas system into new-age methods of genome engineering across various plant species and the impact they have had on tweaking plant genomes and associated outcomes on crop improvement initiatives.

Potential of Genome Editing to Capture Diversity From Australian Wild Rice Relatives

Rice, a staple food worldwide and a model crop, could benefit from the introduction of novel genetics from wild relatives. Wild rice in the AA genome group closely related to domesticated rice is found across the tropical world. Due to their locality outside the range of domesticated rice, Australian wild rice populations are a potential source of unique traits for rice breeding. These rice species provide a diverse gene pool for improvement that could be utilized for desirable traits such as stress resistance, disease tolerance, and nutritional qualities. However, they remain poorly characterized. The CRISPR/Cas system has revolutionized gene editing and has improved our understanding of gene functions. Coupled with the increasing availability of genomic information on the species, genes in Australian wild rice could be modified through genome editing technologies to produce new domesticates. Alternatively, beneficial alleles from these rice species could be incorporated into cultivated rice to improve critical traits. Here, we summarize the beneficial traits in Australian wild rice, the available genomic information and the potential of gene editing to discover and understand the functions of novel alleles. Moreover, we discuss the potential domestication of these wild rice species for health and economic benefits to rice production globally.

Recent advancements in CRISPR/Cas technology for accelerated crop improvement

Precise genome engineering approaches could be perceived as a second paradigm for targeted trait improvement in crop plants, with the potential to overcome the constraints imposed by conventional CRISPR/Cas technology. The likelihood of reduced agricultural production due to highly turbulent climatic conditions increases as the global population expands. The second paradigm of stress-resilient crops with enhanced tolerance and increased productivity against various stresses is paramount to support global production and consumption equilibrium. Although traditional breeding approaches have substantially increased crop production and yield, effective strategies are anticipated to restore crop productivity even further in meeting the world's increasing food demands. CRISPR/Cas, which originated in prokaryotes, has surfaced as a coveted genome editing tool in recent decades, reshaping plant molecular biology in unprecedented ways and paving the way for engineering stress-tolerant crops. CRISPR/Cas is distinguished by its efficiency, high target specificity, and modularity, enables precise genetic modification of crop plants, allowing for the creation of allelic variations in the germplasm and the development of novel and more productive agricultural practices. Additionally, a slew of advanced biotechnologies premised on the CRISPR/Cas methodologies have augmented fundamental research and plant synthetic biology toolkits. Here, we describe gene editing tools, including CRISPR/Cas and its imitative tools, such as base and prime editing, multiplex genome editing, chromosome engineering followed by their implications in crop genetic improvement. Further, we comprehensively discuss the latest developments of CRISPR/Cas technology including CRISPR-mediated gene drive, tissue-specific genome editing, dCas9 mediated epigenetic modification and programmed self-elimination of transgenes in plants. Finally, we highlight the applicability and scope of advanced CRISPR-based techniques in crop genetic improvement.

Occurrence and Nature of Off-Target Modifications by CRISPR-Cas Genome Editing in Plants

CRISPR-Cas-based genome editing allows for precise and targeted genetic modification of plants. Nevertheless, unintended off-target edits can arise that might confer risks when present in gene-edited food crops. Through an extensive literature review we gathered information on CRISPR-Cas off-target edits in plants. Most observed off-target changes were small insertions or deletions (1–22 bp) or nucleotide substitutions, and large deletions (>100 bp) were rare. One study detected the insertion of vector-derived DNA sequences, which is important considering the risk assessment of gene-edited plants. Off-target sites had few mismatches (1–3 nt) with the target sequence and were mainly located in protein-coding regions, often in target gene homologues. Off-targets edits were predominantly detected via biased analysis of predicted off-target sites instead of unbiased genome-wide analysis. CRISPR-Cas-edited plants showed lower off-target mutation frequencies than conventionally bred plants. This Review can aid discussions on the relevance of evaluating off-target modifications for risk assessment of CRISPR-Cas-edited plants.

A cytosine base editor toolkit with varying activity windows and target scopes for versatile gene manipulation in plants

CRISPR/Cas-derived base editing tools empower efficient alteration of genomic cytosines or adenines associated with essential genetic traits in plants and animals. Diversified target sequences and customized editing products call for base editors with distinct features regarding the editing window and target scope. Here we developed a toolkit of plant base editors containing AID10, an engineered human AID cytosine deaminase. When fused to the N-terminus or C-terminus of the conventional Cas9 nickase (nSpCas9), AID10 exhibited a broad or narrow activity window at the protospacer adjacent motif (PAM)-distal and -proximal protospacer, respectively, while AID10 fused to both termini conferred an additive activity window. We further replaced nSpCas9 with orthogonal or PAM-relaxed Cas9 variants to widen target scopes. Moreover, we devised dual base editors with AID10 located adjacently or distally to the adenine deaminase ABE8e, leading to juxtaposed or spaced cytosine and adenine co-editing at the same target sequence in plant cells. Furthermore, we expanded the application of this toolkit in plants for tunable knockdown of protein-coding genes via creating upstream open reading frame and for loss-of-function analysis of non-coding genes, such as microRNA sponges. Collectively, this toolkit increases the functional diversity and versatility of base editors in basic and applied plant research.

Improvement of base editors and prime editors advances precision genome engineering in plants

Clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR associated protein (Cas)-mediated gene disruption has revolutionized biomedical research as well as plant and animal breeding. However, most disease-causing mutations and agronomically important genetic variations are single base polymorphisms (single-nucleotide polymorphisms) that require precision genome editing tools for correction of the sequences. Although homology-directed repair of double-stranded breaks (DSBs) can introduce precise changes, such repairs are inefficient in differentiated animal and plant cells. Base editing and prime editing are two recently developed genome engineering approaches that can efficiently introduce precise edits into target sites without requirement of DSB formation or donor DNA templates. They have been applied in several plant species with promising results. Here, we review the extensive literature on improving the efficiency, target scope, and specificity of base editors and prime editors in plants. We also highlight recent progress on base editing in plant organellar genomes and discuss how these precision genome editing tools are advancing basic plant research and crop breeding.

Genome editing with type II-C CRISPR-Cas9 systems from Neisseria meningitidis in rice

Two type II-C Cas9 orthologs (Nm1Cas9 and Nm2Cas9) were recently identified from Neisseria meningitidis and have been extensively used in mammalian cells, but whether these NmCas9 orthologs or other type II-C Cas9 proteins can mediate genome editing in plants remains unclear. In this study, we developed and optimized targeted mutagenesis systems from NmCas9s for plants. Efficient genome editing at the target with N₄ GATT and N₄ CC protospacer adjacent motifs (PAMs) was achieved with Nm1Cas9 and Nm2Cas9 respectively. These results indicated that a highly active editing system could be developed from type II-C Cas9s with distinct PAM preferences, thus providing a reliable strategy to extend the scope of genome editing in plants. Base editors (BEs) were further developed from the NmCas9s. The editing efficiency of adenine BEs (ABEs) of TadA*-7.10 and cytosine BEs (CBEs) of rat APOBEC1 (rAPO1) or human APOBEC3a (hA3A) were extremely limited, whereas ABEs of TadA-8e and CBEs of Petromyzon marinus cytidine deaminase 1 (PmCDA1) exhibited markedly improved performance on the same targets. In addition, we found that fusion of a single-stranded DNA-binding domain from the human Rad51 protein enhanced the base editing capability of rAPO1-CBEs of NmCas9s. Together, our results suggest that the engineering of NmCas9s or other type II-C Cas9s can provide useful alternatives for crop genome editing.

Human cell based directed evolution of adenine base editors with improved efficiency

Adenine base editors (ABE) are genome-editing tools that have been harnessed to introduce precise A•T to G•C conversion. However, the low activity of ABE at certain sites remains a major bottleneck that precludes efficacious applications. Here, to address it, we develop a directional screening system in human cells to evolve the deaminase component of the ABE, and identify three high-activity NG-ABEmax variants: NG-ABEmax-SGK (R101S/D139G/E140K), NG-ABEmax-R (Q154R) and NG-ABEmax-K (N127K). With further engineering, we create a consolidated variant [NG-ABEmax-KR (N127K/Q154R)] which exhibit superior editing activity both in human cells and in mouse disease models, compared to the original NG-ABEmax. We also find that NG-ABEmax-KR efficiently introduce natural mutations in gamma globin gene promoters with more than four-fold increase in editing activity. This work provides a broadly applicable, rapidly deployable platform to directionally screen and evolve user-specified traits in base editors that extend beyond augmented editing activity.

Efficient generation of homozygous substitutions in rice in one generation utilizing an rABE8e base editor

A new deaminase, TadA8e, was recently evolved in the laboratory. TadA8e catalyzes DNA deamination over 1,000 times faster than ABE7.10. We developed a high-efficiency adenine base editor, rABE8e (rice ABE8e), combining monomeric TadA8e, bis-bpNLS and codon optimization. rABE8e had substantially increased editing efficiencies at NG-protospacer adjacent motif (PAM) and NGG-PAM target sequences compared with ABEmax. For most targets, rABE8e exhibited nearly 100% editing efficiency and high homozygous substitution rates in the specific editing window, especially at Positions A5 and A6. The ability to rapidly generate plant materials with homozygous base substitutions will benefit gene function research and precision molecular breeding.

Precise plant genome editing using base editors and prime editors

The development of CRISPR-Cas systems has sparked a genome editing revolution in plant genetics and breeding. These sequence-specific RNA-guided nucleases can induce DNA double-stranded breaks, resulting in mutations by imprecise non-homologous end joining (NHEJ) repair or precise DNA sequence replacement by homology-directed repair (HDR). However, HDR is highly inefficient in many plant species, which has greatly limited precise genome editing in plants. To fill the vital gap in precision editing, base editing and prime editing technologies have recently been developed and demonstrated in numerous plant species. These technologies, which are mainly based on Cas9 nickases, can introduce precise changes into the target genome at a single-base resolution. This Review provides a timely overview of the current status of base editors and prime editors in plants, covering both technological developments and biological applications.

The CRISPR/Cas9 revolution continues: From base editing to prime editing in plant science

The ability to precisely inactivate or modify genes in model organisms helps us understand the mysteries of life. Clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9), a revolutionary technology that could generate targeted mutants, has facilitated notable advances in plant science. Genome editing with CRISPR/Cas9 has gained great popularity and enabled several technical breakthroughs. Herein, we briefly introduce the CRISPR/Cas9, with a focus on the latest breakthroughs in precise genome editing (e.g., base editing and prime editing), and we summarize various platforms that developed to increase the editing efficiency, expand the targeting scope, and improve the specificity of base editing in plants. In addition, we emphasize the recent applications of these technologies to plants. Finally, we predict that CRISPR/Cas9 and CRISPR/Cas9-based genome editing will continue to revolutionize plant science and provide technical support for sustainable agricultural development.

Novel CRISPR/Cas applications in plants: from prime editing to chromosome engineering

In the last years, tremendous progress has been made in the development of CRISPR/Cas-mediated genome editing tools. A number of natural CRISPR/Cas nuclease variants have been characterized. Engineered Cas proteins have been developed to minimize PAM restrictions, off-side effects and temperature sensitivity. Both kinds of enzymes have, by now, been applied widely and efficiently in many plant species to generate either single or multiple mutations at the desired loci by multiplexing. In addition to DSB-induced mutagenesis, specifically designed CRISPR/Cas systems allow more precise gene editing, resulting not only in random mutations but also in predefined changes. Applications in plants include gene targeting by homologous recombination, base editing and, more recently, prime editing. We will evaluate these different technologies for their prospects and practical applicability in plants. In addition, we will discuss a novel application of the Cas9 nuclease in plants, enabling the induction of heritable chromosomal rearrangements, such as inversions and translocations. This technique will make it possible to change genetic linkages in a programmed way and add another level of genome engineering to the toolbox of plant breeding. Also, strategies for tissue culture free genome editing were developed, which might be helpful to overcome the transformation bottlenecks in many crops. All in all, the recent advances of CRISPR/Cas technology will help agriculture to address the challenges of the twenty-first century related to global warming, pollution and the resulting food shortage.

Base Editing in Plants: Applications, Challenges, and Future Prospects

The ability to create targeted modifications in the genomes of plants using genome editing technologies has revolutionized research in crop improvement in the current dispensation of molecular biology. This technology has attracted global attention and has been employed in functional analysis studies in crop plants. Since many important agronomic traits are confirmed to be determined by single-nucleotide polymorphisms, improved crop varieties could be developed by the programmed and precise conversion of targeted single bases in the genomes of plants. One novel genome editing approach which serves for this purpose is base editing. Base editing directly makes targeted and irreversible base conversion without creating double-strand breaks (DSBs). This technology has recently gained quick acceptance and adaptation because of its precision, simplicity, and multiplex capabilities. This review focuses on generating different base-editing technologies and how efficient they are in editing nucleic acids. Emphasis is placed on the exploration and applications of these base-editing technologies to enhance crop production. The review also highlights the drawbacks and the prospects of this new technology.

Sequence modification on demand: search and replace tools for precise gene editing in plants

Until recently, our ability to generate allelic diversity in plants was limited to introduction of variants from domesticated and wild species by breeding via uncontrolled recombination or the use of chemical and physical mutagens-processes that are lengthy and costly or lack specificity, respectively. Gene editing provides a faster and more precise way to create new variation, although its application in plants has been dominated by the creation of short insertion and deletion mutations leading to loss of gene function, mostly due to the dependence of editing outcomes on DNA repair pathway choices intrinsic to higher eukaryotes. Other types of edits such as point mutations and precise and pre-designed targeted sequence insertions have rarely been implemented, despite providing means to modulate the expression of target genes or to engineer the function and stability of their protein products. Several advancements have been developed in recent years to facilitate custom editing by regulation of repair pathway choices or by taking advantage of alternative types of DNA repair. We have seen the advent of novel gene editing tools that are independent of DNA double-strand break repair, and methods completely independent of host DNA repair processes are being increasingly explored. With the aim to provide a comprehensive review of the state-of-the-art methodology for allele replacement in plants, I discuss the adoption of these improvements for plant genome engineering.

Plant genome editing: ever more precise and wide reaching

Genome-editing technologies consisting of targeted mutagenesis and gene targeting enable us to modify genes of interest rapidly and precisely. The discovery in 2012 of CRISPR/Cas9 systems and their development as sequence-specific nucleases has brought about a paradigm shift in biology. Initially, CRISPR/Cas9 was applied in targeted mutagenesis to knock out a target gene. Thereafter, advances in genome-editing technologies using CRISPR/Cas9 developed rapidly, with base editing systems for transition substitution using a combination of Cas9 nickase and either cytidine or adenosine deaminase being reported in 2016 and 2017, respectively, and later in 2021 bringing reports of transversion substitution using Cas9 nickase, cytidine deaminase and uracil DNA glycosylase. Moreover, technologies for gene targeting and prime editing systems using DNA or RNA as donors have also been developed in recent years. Besides these precise genome-editing strategies, reports of successful chromosome engineering using CRISPR/Cas9 have been published recently. The application of genome editing to crop breeding has advanced in parallel with the development of these technologies. Genome-editing enzymes can be introduced into plant cells, and there are now many examples of crop breeding using genome-editing technologies. At present, it is no exaggeration to say that we are now in a position to be able to modify a gene precisely and rearrange genomes and chromosomes in a predicted way. In this review, we introduce and discuss recent highlights in the field of precise gene editing, chromosome engineering and genome engineering technology in plants.

ABE8e with Polycistronic tRNA-gRNA Expression Cassette Sig-Nificantly Improves Adenine Base Editing Efficiency in Nicotiana benthamiana

Adenine base editor containing TadA8e (ABE8e) has been reported in rice. However, the application of ABE8e in other plant species has not been described, and the comparison between ABE8e and ABE7.10, which is widely used in plants, has also been poorly studied. Here, we developed the ABE8e with the polycistronic tRNA-gRNA expression cassette (PTG-ABE8e) and PTG-ABE7.10 and compared their A-to-G editing efficiencies using both transient and stable transformation in the allotetraploid Nicotiana benthamiana. We found that the editing efficiency of PTG-ABE8e was significantly higher than that of PTG-ABE7.10, indicating that ABE8e was more efficient for A-to-G conversion in N. benthamiana. We further optimized the ABE8e editing efficiency by changing the sgRNA expression cassette and demonstrated that both PTG and single transcript unit (STU) enhanced ABE8e efficiency for A-to-G conversion in N. benthamiana. We also estimated the potential off-target effect of PTG-ABE8e at potential off-targeting sites predicted using an online tool in transgenic plants, and no off-target editing event was found for potential off-targeting sites selected, indicating that ABE8e could specifically facilitate A-to-G conversion. Our results showed that ABE8e with PTG structure was more suitable for A-to-G conversion in N. benthamiana and provided valuable clues for optimizing ABE tools in other plants.

Evolution and Application of Genome Editing Techniques for Achieving Food and Nutritional Security

A world with zero hunger is possible only through a sustainable increase in food production and distribution and the elimination of poverty. Scientific, logistical, and humanitarian approaches must be employed simultaneously to ensure food security, starting with farmers and breeders and extending to policy makers and governments. The current agricultural production system is facing the challenge of sustainably increasing grain quality and yield and enhancing resistance to biotic and abiotic stress under the intensifying pressure of climate change. Under present circumstances, conventional breeding techniques are not sufficient. Innovation in plant breeding is critical in managing agricultural challenges and achieving sustainable crop production. Novel plant breeding techniques, involving a series of developments from genome editing techniques to speed breeding and the integration of omics technology, offer relevant, versatile, cost-effective, and less time-consuming ways of achieving precision in plant breeding. Opportunities to edit agriculturally significant genes now exist as a result of new genome editing techniques. These range from random (physical and chemical mutagens) to non-random meganucleases (MegaN), zinc finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs), clustered regularly interspaced short palindromic repeats (CRISPR)/associated protein system 9 (CRISPR/Cas9), the CRISPR system from Prevotella and Francisella1 (Cpf1), base editing (BE), and prime editing (PE). Genome editing techniques that promote crop improvement through hybrid seed production, induced apomixis, and resistance to biotic and abiotic stress are prioritized when selecting for genetic gain in a restricted timeframe. The novel CRISPR-associated protein system 9 variants, namely BE and PE, can generate transgene-free plants with more frequency and are therefore being used for knocking out of genes of interest. We provide a comprehensive review of the evolution of genome editing technologies, especially the application of the third-generation genome editing technologies to achieve various plant breeding objectives within the regulatory regimes adopted by various countries. Future development and the optimization of forward and reverse genetics to achieve food security are evaluated.

CRISPR/Cas: a Nobel Prize award-winning precise genome editing technology for gene therapy and crop improvement

Since it was first recognized in bacteria and archaea as a mechanism for innate viral immunity in the early 2010s, clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein (Cas) has rapidly been developed into a robust, multifunctional genome editing tool with many uses. Following the discovery of the initial CRISPR/Cas-based system, the technology has been advanced to facilitate a multitude of different functions. These include development as a base editor, prime editor, epigenetic editor, and CRISPR interference (CRISPRi) and CRISPR activator (CRISPRa) gene regulators. It can also be used for chromatin and RNA targeting and imaging. Its applications have proved revolutionary across numerous biological fields, especially in biomedical and agricultural improvement. As a diagnostic tool, CRISPR has been developed to aid the detection and screening of both human and plant diseases, and has even been applied during the current coronavirus disease 2019 (COVID-19) pandemic. CRISPR/Cas is also being trialed as a new form of gene therapy for treating various human diseases, including cancers, and has aided drug development. In terms of agricultural breeding, precise targeting of biological pathways via CRISPR/Cas has been key to regulating molecular biosynthesis and allowing modification of proteins, starch, oil, and other functional components for crop improvement. Adding to this, CRISPR/Cas has been shown capable of significantly enhancing both plant tolerance to environmental stresses and overall crop yield via the targeting of various agronomically important gene regulators. Looking to the future, increasing the efficiency and precision of CRISPR/Cas delivery systems and limiting off-target activity are two major challenges for wider application of the technology. This review provides an in-depth overview of current CRISPR development, including the advantages and disadvantages of the technology, recent applications, and future considerations.

Base editing in rice: current progress, advances, limitations, and future perspectives

Base editing is one of the promising genome editing tools for generating single-nucleotide changes in rice genome. Rice (Oryza sativa L.) is an important staple food crop, feeding half of the population around the globe. Developing new rice varieties with desirable agronomic traits is necessary for sustaining global food security. The use of genome editing technologies for developing rice varieties is pre-requisite in the present scenario. Among the genome editing technologies developed for rice crop improvement, base editing technology has emerged as an efficient and reliable tool for precise genome editing in rice plants. Base editing technology utilizes either adenosine or cytidine base editor for precise editing at the target region. A base editor (adenosine or cytidine) is a fusion of catalytically inactive CRISPR/Cas9 domain and adenosine or cytidine deaminase domain. In this review, authors have discussed the different adenine and cytosine base editors developed so far for precise genome editing of rice via base editing technology. We address the current progress, advances, limitations, as well as future perspectives of the base editing technology for rice crop improvement.

Progression and application of CRISPR-Cas genomic editors

Base editing technology is an efficient tool for genome editing, particularly in the correction of base mutations. Diverse base editing systems were developed according to the dCas9 or nCas9 linked with different deaminase or reverse transcriptase in the editors, including ABEs, CBEs, PEs and dual-functional of base editor (such as CGBE1, A&C-BEmax, ACBE, etc.). Currently, Base editing technology has been widely applied to various fields such as microorganisms, plants, animals and medicine for basic research and therapeutics. Here, we reviewed the advancement of base editing technology. We also discussed the application of base editors in different areas in the future.

Advances in base editing with an emphasis on an AAV-based strategy

The Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)-based base editors have been developed for precisely installing point mutations in genomes with high efficiency. Two editing systems of cytosine base editors (CBEs) and adenine base editors (ABEs) have been developed for conversion of C.G-to-T.A and A.T-to-G.C, respectively, showing the prominence in genomic DNA correction and mutation. Here, we summarize recent optimized approaches in improving base editors, including the evolution of Cas proteins, the choice of deamination enzymes, modification on linker length, base-editor expression, and addition of functional domains. Specifically, in this paper we highlight a strategy of split-intein mediated base-editor reconstitution for its adeno-associated virus (AAV) delivery. The purpose of this article is to offer readers with a better understanding of AAV-mediated base editors, and facilitate them to use this tool in in vivo experiments and potential clinical applications.

Expanding plant genome-editing scope by an engineered iSpyMacCas9 system that targets A-rich PAM sequences

The most popular CRISPR-SpCas9 system recognizes canonical NGG protospacer adjacent motifs (PAMs). Previously engineered SpCas9 variants, such as Cas9-NG, favor G-rich PAMs in genome editing. In this manuscript, we describe a new plant genome-editing system based on a hybrid iSpyMacCas9 platform that allows for targeted mutagenesis, C to T base editing, and A to G base editing at A-rich PAMs. This study fills a major technology gap in the CRISPR-Cas9 system for editing NAAR PAMs in plants, which greatly expands the targeting scope of CRISPR-Cas9. Finally, our vector systems are fully compatible with Gateway cloning and will work with all existing single-guide RNA expression systems, facilitating easy adoption of the systems by others. We anticipate that more tools, such as prime editing, homology-directed repair, CRISPR interference, and CRISPR activation, will be further developed based on our promising iSpyMacCas9 platform.

Sophisticated CRISPR/Cas tools for fine-tuning plant performance

Over the last years, the discovery of various natural and the development of a row of engineered CRISPR/Cas nucleases have made almost every site of plant genomes accessible for the induction of specific changes. Newly developed tools open up a wide range of possibilities for the induction of genetic variability, from changing a single bp to Mbps, and thus to fine-tune plant performance. Whereas early approaches focused on targeted mutagenesis, recently developed tools enable the induction of precise and predefined genomic modifications. The use of base editors allows the substitution of single nucleotides, whereas the use of prime editors and gene targeting methods enables the induction of larger sequence modifications from a few bases to several kbp. Recently, through CRISPR/Cas-mediated chromosome engineering, it became possible to induce heritable inversions and translocations in the Mbp range. Thus, a novel way of breaking and fixing genetic linkages has come into reach for breeders. In addition, sequence-specific recruitment of various factors involved in transcriptional and post-transcriptional regulation has been shown to provide an additional class of methods for the fine tuning of plant performance. In this review, we provide an overview of the most recent progress in the field of CRISPR/Cas-based tool development for plant genome engineering and try to evaluate the importance of these developments for breeding and biotechnological applications.

Genome editing for plant research and crop improvement

The advent of clustered regularly interspaced short palindromic repeat (CRISPR) has had a profound impact on plant biology, and crop improvement. In this review, we summarize the state-of-the-art development of CRISPR technologies and their applications in plants, from the initial introduction of random small indel (insertion or deletion) mutations at target genomic loci to precision editing such as base editing, prime editing and gene targeting. We describe advances in the use of class 2, types II, V, and VI systems for gene disruption as well as for precise sequence alterations, gene transcription, and epigenome control.

CRISPR base editing and prime editing: DSB and template-free editing systems for bacteria and plants

CRISPR-Cas (Clustered Regularly Interspaced Short Palindromic Repeats-CRISPR associated) has been extensively exploited as a genetic tool for genome editing. The RNA guided Cas nucleases generate DNA double-strand break (DSB), triggering cellular repair systems mainly Non-homologous end-joining (NHEJ, imprecise repair) or Homology-directed repair (HDR, precise repair). However, DSB typically leads to unexpected DNA changes and lethality in some organisms. The establishment of bacteria and plants into major bio-production platforms require efficient and precise editing tools. Hence, in this review, we focus on the non-DSB and template-free genome editing, i.e., base editing (BE) and prime editing (PE) in bacteria and plants. We first highlight the development of base and prime editors and summarize their studies in bacteria and plants. We then discuss current and future applications of BE/PE in synthetic biology, crop improvement, evolutionary engineering, and metabolic engineering. Lastly, we critically consider the challenges and prospects of BE/PE in PAM specificity, editing efficiency, off-targeting, sequence specification, and editing window.

Applications of CRISPR-Cas in agriculture and plant biotechnology

The prokaryote-derived CRISPR-Cas genome editing technology has altered plant molecular biology beyond all expectations. Characterized by robustness and high target specificity and programmability, CRISPR-Cas allows precise genetic manipulation of crop species, which provides the opportunity to create germplasms with beneficial traits and to develop novel, more sustainable agricultural systems. Furthermore, the numerous emerging biotechnologies based on CRISPR-Cas platforms have expanded the toolbox of fundamental research and plant synthetic biology. In this Review, we first briefly describe gene editing by CRISPR-Cas, focusing on the newest, precise gene editing technologies such as base editing and prime editing. We then discuss the most important applications of CRISPR-Cas in increasing plant yield, quality, disease resistance and herbicide resistance, breeding and accelerated domestication. We also highlight the most recent breakthroughs in CRISPR-Cas-related plant biotechnologies, including CRISPR-Cas reagent delivery, gene regulation, multiplexed gene editing and mutagenesis and directed evolution technologies. Finally, we discuss prospective applications of this game-changing technology.

Precision Breeding Made Real with CRISPR: Illustration through Genetic Resistance to Pathogens

Since its discovery as a bacterial adaptive immune system and its development for genome editing in eukaryotes, the CRISPR technology has revolutionized plant research and precision crop breeding. The CRISPR toolbox holds great promise in the production of crops with genetic disease resistance to increase agriculture resilience and reduce chemical crop protection with a strong impact on the environment and public health. In this review, we provide an extensive overview on recent breakthroughs in CRISPR technology, including the newly developed prime editing system that allows precision gene editing in plants. We present how each CRISPR tool can be selected for optimal use in accordance with its specific strengths and limitations, and illustrate how the CRISPR toolbox can foster the development of genetically pathogen-resistant crops for sustainable agriculture.

Genome editing with the CRISPR-Cas system: an art, ethics and global regulatory perspective

Over the last three decades, the development of new genome editing techniques, such as ODM, TALENs, ZFNs and the CRISPR-Cas system, has led to significant progress in the field of plant and animal breeding. The CRISPR-Cas system is the most versatile genome editing tool discovered in the history of molecular biology because it can be used to alter diverse genomes (e.g. genomes from both plants and animals) including human genomes with unprecedented ease, accuracy and high efficiency. The recent development and scope of CRISPR-Cas system have raised new regulatory challenges around the world due to moral, ethical, safety and technical concerns associated with its applications in pre-clinical and clinical research, biomedicine and agriculture. Here, we review the art, applications and potential risks of CRISPR-Cas system in genome editing. We also highlight the patent and ethical issues of this technology along with regulatory frameworks established by various nations to regulate CRISPR-Cas-modified organisms/products.

Directed evolution of adenine base editors with increased activity and therapeutic application

The foundational adenine base editors (for example, ABE7.10) enable programmable A•T to G•C point mutations but editing efficiencies can be low at challenging loci in primary human cells. Here we further evolve ABE7.10 using a library of adenosine deaminase variants to create ABE8s. At NGG protospacer adjacent motif (PAM) sites, ABE8s result in ~1.5× higher editing at protospacer positions A5-A7 and ~3.2× higher editing at positions A3-A4 and A8-A10 compared with ABE7.10. Non-NGG PAM variants have a ~4.2-fold overall higher on-target editing efficiency than ABE7.10. In human CD34+ cells, ABE8 can recreate a natural allele at the promoter of the γ-globin genes HBG1 and HBG2 with up to 60% efficiency, causing persistence of fetal hemoglobin. In primary human T cells, ABE8s achieve 98-99% target modification, which is maintained when multiplexed across three loci. Delivered as messenger RNA, ABE8s induce no significant levels of single guide RNA (sgRNA)-independent off-target adenine deamination in genomic DNA and very low levels of adenine deamination in cellular mRNA.

Genome Editing Technologies for Rice Improvement: Progress, Prospects, and Safety Concerns

Rice (Oryza sativa) is an important staple food crop worldwide; to meet the growing nutritional requirements of the increasing population in the face of climate change, qualitative and quantitative traits of rice need to be improved. Stress-tolerant crop varieties must be developed with stable or higher yields under stress conditions. Genome editing and speed breeding have improved the accuracy and pace of rice breeding. New breeding technologies including genome editing have been established in rice, expanding the potential for crop improvement. Recently, other genome editing techniques such as CRISPR-directed evolution, CRISPR-Cas12a, and base editors have also been used for efficient genome editing in rice. Since rice is an excellent model system for functional studies due to its small genome and close syntenic relationships with other cereal crops, new genome-editing technologies continue to be developed for use in rice. In this review, we focus on genome-editing tools for rice improvement to address current challenges and provide examples of genome editing in rice. We also shed light on expanding the scope of genome editing and systems for delivering homology-directed repair templates. Finally, we discuss safety concerns and methods for obtaining transgene-free crops.

Development of Plant Prime-Editing Systems for Precise Genome Editing

Prime-editing systems have the capability to perform efficient and precise genome editing in human cells. In this study, we first developed a plant prime editor 2 (pPE2) system and test its activity by generating a targeted mutation on an HPT^-ATG reporter in rice. Our results showed that the pPE2 system could induce programmable editing at different genome sites. In transgenic T₀ plants, pPE2-generated mutants occurred with 0%-31.3% frequency, suggesting that the efficiency of pPE2 varied greatly at different genomic sites and with prime-editing guide RNAs of diverse structures. To optimize editing efficiency, guide RNAs were introduced into the pPE2 system following the PE3 and PE3b strategy in human cells. However, at the genomic sites tested in this study, pPE3 systems generated only comparable or even lower editing frequencies. Furthemore, we developed a surrogate pPE2 system by incorporating the HPT^-ATG reporter to enrich the prime-edited cells. The nucleotide editing was easily detected in the resistant calli transformed with the surrogate pPE2 system, presumably due to the enhanced screening efficiency of edited cells. Taken together, our results indicate that plant prime-editing systems we developed could provide versatile and flexible editing in rice genome.